Received 19 April 2012Accepted 10 May 2012Published online 22 May 2012

Acknowledgments:

This work was carried out under the COSMOS project which is funded by the Engineering and Physical Sciences Research Council UK (Grants No. EP/D04894X/1 and No. EP/H046658/1). S.M.M. acknowledges The Royal Society for financial support. F.C. thanks R. James for useful discussions.

32. For example, the sign of (e1 + e3) for 5CB is reported as negative in Ref. 22, and positive in Ref. 24.

33.

33. The value of |e3| for a certain bent-core material has been reported as 15.8 pC/m in Ref. 75 and as 62 000 pC/m in Ref. 31. However, this latter value has been shown theoretically to be essentially impossible.76

58. The calculation required a value of the twist elastic constant, for which K2 = 3.3 pN was used (at a reduced temperature of ≈0.98 to be consistent with our other input values) from Ref. 61: although the essential result is not particularly sensitive to the exact value of K2 used.

71. The temperature as determined by the readout of the hot-stage controller is precise (to the nearest 0.01°C), but not necessarily accurate. However, the value of Tc as determined using a hot-stage and hot-stage controller with a Fréedericksz cell was 35.51 °C, which supports the supplier's value.